In fiber optic technology there are many instances where it is necessary to optically align and optically couple a bulk element such as a semiconductor device and/or a micro electromechanical system (MEMs) to an optical component located on a planar lightwave circuit (PLC). A PLC employs planar optical integration to manufacture waveguide circuits on silicon wafers, using processing techniques similar to those used in the silicon microelectronics industry. Doped-silica waveguides are usually preferred because they have a number of attractive properties including low cost, low loss, stability, and compatibility for coupling to laser diodes, other waveguides, high NA fiber and standard fiber. Such a waveguide is fabricated on a carrier substrate, which typically comprises silicon or silica. The cost of achieving proper alignment between the bulk element and the PLC is often high because it involves the use of expensive lenses and processes in order to prevent light from being lost as its passes between the components being aligned. The coupling efficiency of the light, which is highly dependent on accurate alignment, should generally be maximized since this helps offset the many ways light can be lost in a PLC. Moreover, when the optical system being aligned is designed to transmit single mode light, high efficiency coupling can be particularly hard to achieve because the tolerance to misalignment is so great.
Some of the most common bulk elements used in fiber optics are active semiconductor devices such as lasers, light emitting diodes, and semiconductor optical amplifiers. Passive bulk elements that are employed include such semiconductor devices as detectors and filters as well as other optical elements such as fiber. For these devices to be useful, there must an alignment mechanism to optically couple them with the PLC.
The mechanical tolerance for adequate optical alignment in such systems is severe since these devices are small and are affected by the modal properties of the light. The most difficult systems to align involve semiconductor laser-based devices. A semiconductor laser that is to be optically aligned to a single-mode waveguide, which is the type commonly used in optical telecommunication systems, has a typical positional misalignment tolerance of less than about one micron, and typically on the order of about a half a micron.
Typically, a semiconductor laser is aligned to an optical fiber or PLC by either a soldering or welding technique. If an active alignment technique is used, an optical signal is transmitted through the components and detected. The alignment is performed by manipulating the optical fiber or PLC so that the transmission characteristics result in the highest possible performance level for the system, which usually is achieved when the coupling efficiency is maximized.
The process of active alignment can be costly and time consuming because the alignment tolerance is so tight that in the course of bonding the optical components can shift or move, thereby causing alignment impairments. For example, when welding is employed distortion often arises that causes a shift in position, necessitating realignment of the components by bending them back into position after the bond is formed. Similarly, when soldering is employed, the solder usually shrinks upon cooling, causing some shift in the position of the components.
In a conventional alignment process in which a semiconductor device such as a laser is to be attached and optically aligned with an optical component such as an optical fiber, the semiconductor device is first bonded to a substrate. A weldable fixture is also attached to substrate. The optical fiber is secured to the weldable fixture. After securing the fiber to the fixture, the fixture is physically manipulated to achieve the desired coupling efficiency. The semiconductor device is not moved during the alignment process because it is electrically connected and thermally contacted for heat sinking to maintain stability. As a result the laser cannot be moved on the bonded substrate. This technique requires specialized fixtures and flexures to enable bending in order to move the optical fiber back into position. The fixtures may consist of special fiber mounts that allow the fiber to be optically coupled to components such as a laser or a PLC. The flexures are complex structures enabling bending in various directions, and are sometimes manufactured by a LIGA process, which is a relatively expensive electroplating process that uses a thick photoresist specially patterned by exposure to synchrotron radiation.
Typical state-of-the-art device alignment techniques require industrial lasers to provide laser welding or laser soldering. These industrial lasers are expensive, and require special designs within the product, including extra piece parts, in order to implement the bonding and alignment processes. Apart from the added cost of the additional parts and processing equipment, these extra piece parts can substantially increase the size of the overall assembly.